"[Her schedule] will permit very little," he explained. "But if we can get a hold of her anytime, we will."

However, Whedon admitted that he would consider recruiting actors from his past shows, including Buffy, Firefly and Dollhouse.

"You look for the best person for the role, and if that person is someone you already know, then great," he said. "But as much as I would love to - and once did - make a movie just with my friends, you always want to increase the family."

Clark Gregg stars in S.H.I.E.L.D. as Agent Coulson.

Whedon added that S.H.I.E.L.D. - which is yet to be officially picked up for a series - will be a "very hopeful show".

"It's not about murder, and it's not about crime, and it's not people looking into their own belly buttons," he claimed. "It's about people who are trying to help each other, and that's one of the things I loved about comic books.

"They had costumes and the villains were cool [but] they stood for something, and I like doing a show that does that. We're trying very hard to be true to [Marvel's] ethos and also to the structure of their universe, so it is definitely a Marvel show."